Field effect transistors with lateral channel design attract the power electronics design engineers as they offer much faster switching as compared to vertically built Si and SiC power field effect transistors (FETs). For example, Gallium Nitride based Heterostructure Field Effect Transistors (HFETs) demonstrate exceptional potential for power electronics applications as high voltage, high power switches. Since the first demonstration of high voltage operation [G. Simin, X. Hu, N. Ilinskaya et al, “7.5 kW/mm2 current switch using AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors on SiC substrates”, Electronics Lett., V. 36, No. 24, pp. 2043-2044, 2000.], the breakdown voltages up to 1.5-2 kV have been reported, depending on the gate-drain spacing and surface conditions [N. Tipirneni, A. Koudymov, V. Adivarahan et al, “The 1.6 kV AlGaN/GaN HFETs”, IEEE Electron Device Letters, V. 27, N9, 716-718, September 2006; S. G. Pytel, S. Lentijo, A. Koudymov et al, “AlGaN/GaN MOSHFET integrated circuit power converter”, Proc. IEEE Power Electronics Specialists Conference (PESC'04), pp. 579-584, 2004]. While most of the reports mention the values of blocking voltages and ON resistances well above the performance of other semiconductor devices, very few demonstrations of the dynamic operation of such devices are published yet, mostly limiting the voltage range and switching frequency to low values. At sufficiently high frequencies exceeding 10-100 kHz, the dynamic breakdown voltage significantly degrades leading to the premature device blow up; and so called frequency dispersion, or current collapse [A. F. M. Anwar, S. Islam, and R. Webster, “Carrier Trapping and Current Collapse Mechanism in GaN Metal-Semiconductor Field Effect Transistors”, Applied Phys. Lett., V. 84, No. 11, pp. 1970-1972, March 2004; S. Nozaki, H. Feick, E. R. Weber et al, “Compression of the DC Drain Current by Electron Trapping in AlGaN/GaN Modulation Doped Field-Effect Transistors”, Applied Phys. Lett., V. 78, No. 19, pp. 2896-2898, May 2001], affects the ON resistance.
For the purpose of present invention, we will refer to the experimental results of one of the most complete experimental studies of the HFET breakdown voltages [G. Simin, N. Tipirneni, S. Rai et al, “1.5 kV Power AlGaN/GaN HFETs”, 2005 International Semiconductor Device Research Simposium, ISDRS'2005 Abstract Book, pp. 164-165, December 2005]. In this study, the breakdown voltage dependences on the drain bias were compared for the HFETs with bare surfaces, silicon nitride passivation and utilizing the field plates. While in case of bare surface, the breakdown voltage was increasing nearly linearly with the gate-drain spacing LGD, reaching record high 1.6-1.8 kV with LGD=20 μm, after SiN passivation it decreased to spacing-independent 35-40 V, slightly recovering with the help of field plates.
Although the result for bare surface HFETs looks much more encouraging for power electronics, it does not find an explanation within traditional concept of the electric field distribution in the HFET channel (see, for example, [S. Karmalkar and N. Soudabi, “A Closed-Form Model of the Drain-Voltage Dependence of the OFF-State Channel Electric Field in a HEMT with a Field Plate”, IEEE Trans. Electron Dev., V. 53, No. 10, October 2006; K. Kosaka, T. Fujishima, K. Inoue et al, Temperature distribution analysis of AlGaN/GaN HFETs operated around breakdown voltage using micro-Raman spectroscopy and device simulation, Physica Status Solidi (c), V. 4, No. 7, pp. 2744-2747, June 2007]). According to this concept, the electric field has a peak near the gate edge and decreases towards the drain with the slope that is nearly independent on the applied voltage, similarly to a p-n junction, in such a way that for long enough gate-drain spacing the peak value reaches the critical value for the breakdown, in accordance to the experimental data for SiN passivation. It was suggested in [G. Simin, N. Tipirneni, S. Rai et al, “1.5 kV Power AlGaN/GaN HFETs”, 2005 International Semiconductor Device Research Simposium, ISDRS'2005 Abstract Book, pp. 164-165, December 2005] that in the case of bare surface, the electric field distribution along the channel is way different from this traditional concept. The two-gate measurement was performed in order to determine the actual channel voltage distribution, where the gate closest to the device source was used to control the channel, while the second gate was used as a probe electrode. The voltage sweep was applied to the second gate, and leakage current through it was detected. By zeroing the leakage current, the potential of the second gate was made equal to the channel potential beneath it. Studying the devices with different spacing between the first and the second gate, the authors of [G. Simin, N. Tipirneni, S. Rai et al, “1.5 kV Power AlGaN/GaN HFETs”, 2005 International Semiconductor Device Research Simposium, ISDRS'2005 Abstract Book, pp. 164-165, December 2005] were able to reconstruct the channel potential distribution.
It was shown that the depletion region associated with high electric field domain in case of the bare surface HFET extends unexpectedly long at relatively low biases, reaching 2 μm for ˜50 V and 6 μm for 75-100 V of drain bias. The authors suggested that such long depletion extension is somehow related to the charge at the HFET surface, however did not give detailed explanation.
The present invention provides an approach and method of designing the high voltage blocking FETs with lateral channel by providing an additional path for the access region recharge during the FET switching. It will be demonstrated that the access region recharge is a necessary component in achieving fast and reliable switching and voltage blocking.